Forming dies for stamping sheet metal parts have been traditionally made of high strength tool steel because of its rigidity and durability. In the automotive industry, forming dies made of tool steel have been used to stamp out automobile body parts from steel sheets. However, this type of stamping dies is very expensive to make due to the extensive machining required to make them.
It is a common practice in the automotive industry that before a new vehicle having a new body style is put in production, a limited number of concept cars or prototype vehicles is first built for testing. Designing forming tools with tool steel for stamping sheet metal parts used in these prototype vehicles would not be practical for several reasons. First, a prototype vehicle has to be built in a relatively short time which prohibits the use of tool steel for stamping dies due to the extensive machining required. Secondly, the design of a prototype vehicle is frequently changed many times from its original design before it reaches a final production model. This means that many stamping dies would have to be built before the design of a sheet metal part is finalized thus making the building of stamping dies with tool steel prohibitive for cost reasons.
A method of making prototype stamping tools is by using a zinc alloy material called Kirksite. Even though a Kirksite tool is more economical to make because of the lower cost of the material, a Kirksite tool still requires the use of a foundry as well as extensive machining to obtain the desired contour and close match between the tool halves. Furthermore, a Kirksite tool is extremely heavy and very difficult to handle.
In recent years, there has been a renewed effort to develop mass castable polymeric materials to make durable sheet metal forming tools. One of these commonly used materials is epoxy.
It is well known in the art that it is extremely difficult to formulate a mass castable, room temperature fast curing epoxy that can be cast-to-size into high strength tools. by mass castable, we mean the pouring of a liquid epoxy composition in one casting step to produce a completed portion of a tool. By cast-to-size, we mean a process in which the exact contour is obtained on the surface of a tool such that no machining, barbering, or spotting on the tool surface is required.
For instance, U.S. Pat. No. 4,423,094 to Dearlove et al disclosed a tough, durable bisphenol A epoxy composition for use in making sheet metal stamping dies. While this material exhibits good mechanical strength, it does not form a durable punch for stamping tools. Moreover, it requires an extensive curing procedure, i.e. it must be hardened at room temperature overnight and then post-cured at 150.degree. C. for two hours. Since most stamping tools for automobile body panels are large in size and weight, it is preferable to make dies that need no elevated temperature curing.
Others have used room temperature curable epoxy resin systems such as those based on bisphenol A and an aromatic amine catalyst to make plastic tools. An extensive curing period between 4 days to 1 week is required for this type of room temperature curable epoxy composition. To achieve rapid cure, i.e., to cure in less than 24 hours, an aliphatic amine catalyst instead of an aromatic amine catalyst must be used in an epoxy molding composition. This type of rapid curing epoxy system has been used in adhesives and coatings where the maximum thickness of the epoxy layer is no more than 1/4 inch thick.
Attempts to use this type of rapid curing epoxy composition in bulk casting epoxy tools have met no success. The major problems encountered in casting a bulk section epoxy tooling with a rapid curing epoxy composition are severe shrinkage and dimensional distortion which leads to unacceptable warpage of the tool. These problems are caused by the extreme heat generated by the exothermic curing process. Since the curing process proceeds very rapidly, the amount of exothermic heat cumulated in a bulk section epoxy tooling cannot be rapidly dissipated by heat transfer due to the low thermal conductivity of epoxy. This large amount of exothermic heat causes the formation of localized heat pockets and thermal shocks which lead to shrinkage, cracking, and dimensional distortions. Therefore, no one skilled in the art has been able to use the combination of bisphenol A epoxy resin and aliphatic amine catalysts in making high strength epoxy tools that are mass-cast, cast-to-size, and fast cured.
When a material is selected for building stamping tools, both its compressive strength and its tensile strength are important considerations. To sustain a high compressive load in the vertical direction, a tooling material must have high compressive strength. Similarly, to sustain a high tensile load in the horizontal direction, a tooling material must also have high tensile strength. This type of tensile load, for example, is frequently seen in the cavity of a stamping tool having a V-shaped concave configuration in the tool surface. Most commercially available epoxy tooling materials do not have the necessary combination of compressive strength and tensile strength for making stamping tools.
For instance, we have tested one of the widely used mass-castable epoxy tooling materials, Magnolia 6012, and found it has a tensile strength of 35.7 MPa, a compressive strength of 74.0 MPa, and a flexural strength of 62.7 MPa. It requires an extensive length of time, i.e. 4 days, to cure while producing a 3% volume shrinkage. We have learned from our experience using this material that, for some complex tools with the V-shaped concave configuration, its mechanical properties were unacceptable. It is believed that the tooling material should have a minimum tensile strength of 40 MPa and a minimum compressive strength of 100 MPa.
It is therefore an object of the present invention to provide a mass-cast epoxy tooling composition that has high compressive strength and high tensile strength sufficient for making durable stamping tools.
It is another object of the present invention to provide an epoxy tooling composition that can be rapidly cured at room temperature in less than 24 hours without dimensional distortion.
It is yet another object of the present invention to provide an epoxy tooling composition that can be rapidly cured at room temperature in less than 24 hours to make a cast-to-size metal sheet forming die having optimum dimensional stability.
It is a further object of the present invention to provide an epoxy tooling composition that can be rapidly cured at room temperature in less than 24 hours into a dimensionally stable sheet metal forming tool having high strength and durability.